Learn the circle of fifths. Generally represented as a circle graph with key signatures written out for reference, the circle of fifths starts at C and progresses through the chromatic scale by fifths when you follow the circle clockwise and fourths when you follow the circle counterclockwise. You can purchase a circle of fifths chart at a local music store or find one online for free (see Resources).
Start on C at the top of the circle of fifths and count up a fifth (five notes including the C). This will bring you to G. Remember that the key of C has no sharps or flats and you will add an F# to the key of G. These are the only two things you need to remember upfront. The circle of fifths tells you everything else.
To find the next key after G, count up a fifth from G (clockwise) and you come to D. The key of D has two sharps: the F# from the previous key, plus an added sharp a fifth above the last sharp added, which was the F#. This means that the key of D has an F# and a C#. A fifth above D brings you to the next major key, which is A. Keep the previous two sharps of F# and C# and add a third sharp a fifth above the last sharp added, which was C#. The key of A, then, will have F#, C#, and G#. This pattern continues all the way around the circle and allows you to figure out the major key signatures.
Follow the circle of fifths backward (counterclockwise), starting at C, but instead of moving in fifths, you will move in fourths. From C, the next major scale is F. Remember that it has one flat note, which is a Bb. Now all you need to do is count in fourths to bring you to the next major key, which is Bb, and add a new flat note to the previous by counting a fourth counterclockwise. Continue to do this counterclockwise, the same way you did it clockwise.
Find the sixth note of each major key to identify the related minor key. The related minor key of the C major scale is A minor, the sixth note of the C major scale.